virtio_net: bugfix overflow inside xdp_linearize_page()

Here we copy the data from the original buf to the new page. But we
not check that it may be overflow.

As long as the size received(including vnethdr) is greater than 3840
(PAGE_SIZE -VIRTIO_XDP_HEADROOM). Then the memcpy will overflow.

And this is completely possible, as long as the MTU is large, such
as 4096. In our test environment, this will cause crash. Since crash is
caused by the written memory, it is meaningless, so I do not include it.

Fixes: 72979a6c ("virtio_net: xdp, add slowpath case for non contiguous buffers")
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>